1. Field of the Invention
The invention relates to the in situ coating of the internal surfaces of pipelines. More particularly, it relates to an in situ coating system overcoming the disadvantages associated with conventional in situ coating techniques.
2. Description of the Prior Art
The in-situ coating of the internal surfaces of pipelines, using the double plug extrusion approach, has been practiced basically without change since its inception in the early 1940's. The standard procedure employed comprises trapping a solid slug of coating material between two pigs (or plugs) and moving this pig system through the pipeline by means of the force produced by a differential pressure imposed across the pig system. As the double pig system moves through the pipeline to be coated, the coating material is extruded onto the internal pipe surface by the trailing pig.
In the practice of this standard procedure, a two part epoxy coating composition is batch mixed at the inlet of the pipeline to be coated. The coating composition consists of a coating resin component and a hardener component. The hardener is manually poured into the resin until the proper volume ratio of components is achieved. The resulting mixture is then blended by hand stirring, or by means of an impeller driven by a pneumatic motor. The thus blended mixture is then hand poured, or pumped through a hose into the pipeline at the inlet end thereof between the leading and trailing pigs of the double pig system.
This batch mixing procedure requires frequent movement of material storage drums, typically weighing about 650 lbs., and the hand lifting of 5 gallon pails weighing about 45 lbs. During the practice of this procedure, coating spills, dripping and splashing often occur as the result of the removal and insertion of awkward mixing and pumping devices into the coating material containers. In addition, the containers must remain open for the mixing and pumping operations, thus causing direct exposure of operating personnel to solvent vapors.
Batch mixing and injection as employed in the standard practice are also time consuming operations. This time factor is particularly important, and becomes critical, in hot weather conditions, under which the so called "pot life" of the coating composition is shortened. The undesirable introduction of air bubbles into the coating compositions also occurs in the course of the customary hand mixing or mixing by the use of impeller means.
In addition to such practical operating problems and concerns at the inlet end of the pipe, the standard procedure is also confronted with practical problems and concerns at the outlet end of the pipe. The standard procedure at said outlet end is to manually adjust a discharge throttle valve to control back pressure as the dual pig system, or pig train, moves through the pipeline to be coated. A discharge hose is connected to the pipeline to remove excess coating material therefrom, said hose typically being connected to the pipeline at a point upstream of the point at which the leading pig of the pig system is stopped by a suitable bumper or gate. As the trailing pig continues to move in the direction of the then stationary leading pig, excess coating material is forced from the pipeline through said discharge hose. The discharge throttle valve is typically positioned in the discharge hose line that extends from the pipeline to an open drum used to collect excess coating material. Two operators are usually required for carrying out the necessary operations at the outlet end of the pipeline. The operator is needed to control the discharge throttle valve, while the other holds the discharge hose in the open drum being used to collect the excess coating material. The flow of coating material and of associated gas through the hose has a tendency to whip the hose around in a manner making such manual attention necessary from a practical operating viewpoint. Before the coating pig stream reaches the downstream end of the pipeline, a pressurized gas flow will be discharged through the discharge throttle valve and from the discharge hose. When the pig train arrives at said downstream end of the pipe, and the leading pig is stopped, the flow through the discharge hose changes abruptly from pressurized gas flow to excess coating material discharge. As a result, such operations are typically accompanied by the splashing and spilling of excess coating material and the exposure of the operators to solvent vapors.
While the in situ coating of pipelines is a highly desirable, and even essential, approach to necessary pipeline maintenance, it will thus be appreciated that numerous practical operating problems and disadvantages are associated with the standard procedure employed in the art for using two component coating compositions and dual pig systems. There is a genuine need in the art to overcome such problems and disadvantages not only to enable the in situ coating operation to be carried out more efficiently at a jobsite, with greater protection of operating personnel from environmental hazards and from the potential for vapor ignition and explosive conditions, but also to enable the effectiveness of the in situ coating operation itself to be enhanced.
It is an object of the invention, therefore, to provide an improved system for the in situ coating of pipelines.
It is another object of the invention to provide a system for the proportioning and mixing of two component coating compositions effectively avoiding solvent vapor exposure and coating material spills and drips at the inlet end of the pipeline to be coated.
It is another object of the invention to provide an in-situ pipeline coating system in which the proportioning and mixing of the components of a two component coating composition, and the injection thereof into the pipeline to be coated, can be carried out without the undesirable introduction of air bubbles into the coating composition.
It is a further object of the invention to provide an in situ coating system in which the passage of the dual pig system through the pipeline can be effectively controlled and excess coating material can be effectively discharged at the downstream end of the pipeline without exposure of operating personnel to pressurized gas, solvent vapors, and coating material splash.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.